48.下列哪個函數的定義域恰為所有「正實數」? (A)\(f_1(x) = \frac{1}{3} \log_{10}(2x)\)(B)f2 (x)=2sin(x − π ) (C)\(f_3(x) = \frac{2}{x}\)(D)f4 (x)=√5x